Problems with Solidify Modifier and Model Shape
I checked the file, the subdivision surface modifier is first then the solidify modifier. Also there were some extra (double) vertices that messed up the modifiers.

Solution:


Deactivate all modifiers
Go in edit Mode and select all with A
Press Alt+M "by Distance" (should remove 12 vertices)
Move solidify modifier to top with the small arrow
Select the object and go in object mode to RMB and select smooth shade
Go in the "Object Data Properties Panel" the green upside down triangle
Unfold the "Normals" section and activate "Auto smooth" default value of 30 is fine
Smile


The surface now looking better, the thickness can be slightly reduced as the subD modifier works over it, so you can adjust it a bit by increasing the offset in the solidify modifier.

Also the offset of the solidify modifier is working based on the normals of the objects faces, which should normally face outwards.
Using a positive offset you make your object thicker then it was, negative offset it gains thickness inwards.

Your main problem with the subD modifier is though that you would need to have a control-loop at the bottom to ensure the form/thickness.

Here the file with a control-loop at the bottom.